[AMRadio] QST AM Article and SSB Power


Donald Chester k4kyv at charter.net
Fri Feb 12 23:22:13 EST 2010


The article "Why Not Give AM a Try" that appeared in February QST (p. 43) points out a not-so-well-known fact that will undoubtedly be overlooked by most readers:

(Quote)
Power level

Your 100 W SSB transceiver puts out 100 W PEP on SSB.  That is, at the peak of your voice envelope the  power output is 100 W, just as it can be 100 W with your key down on CW.  When you're not talking on SSB, your power output is 0 W.  The actual average output is typically less than 10W, depending on voice characteristics. (end quote)

Of course, this is in reference to a good quality SSB voice signal that is not over-driving the rig or linear amp, and is not overly processed.

At the 10w level, on an amplifier capable of 100 watts peak, the amplitude of the signal voltage is running at just 31.6% of the amplifier's peak capability.  The amplifier is running at 1/10 of its peak power capability, and since power is a function of the square of signal voltage, the output voltage is 1/√10= 1/3.1623 = .316 or 31.6% of the peak voltage capability.

If you ever watched the analogue VU meters at a recording studio or broadcast station, you might have noticed that when a person is talking, the meter hovers at about the -10 VU or 30% modulation level, even though it may kick up towards 0 VU (100% modulation) on occasional peaks.  With music and other program material, it may run much higher.

So, the average power output level of a linear amplifier running SSB is actually less than the unmodulated carrier power of that same amplifier running AM at the same peak power level, and the SSB efficiency is averaging considerably less than when the amplifier is running AM.  The difference is not so obvious, because with AM, the carrier operates at a continuous 100% duty cycle, while with SSB, the power output is intermittent, running at a very low duty cycle, but the meters kick up to maximum on voice peaks while the meter stands still when the amplifier is operated properly on AM.

So much for the alleged "inefficiency" of a linear amplifier when it is used to run AM.

This means that at 1500 watts pep output, the average output power of a SSB transmitter should not exceed about 150 watts.

Therefore, the p.e.p. power limit penalised SSB just as much or even more so than it did AM, assuming that the SSB transmitter is run cleanly and properly.  Under the old DC input rule, a linear could be run up to where the meters indicated a kilowatt DC average input, with the peaks allowed to go as high as the headroom of the amplifier permitted. Amplifiers in the 50's-70's were universally touted in the ads as capable of "2000 watts (pep in tiny letters) input".  By running the linear at higher peak power, the clean SSB output power could be legally run considerably higher than it can under the present rules.

How many of the slopbucket linears you hear on the air these days are really operated within that power limitation?

Now, explain that to the next slopbucketeer you hear P & M'ing that AM'ers run "illegal power".  

(Unfortunately, the concept will probably go right over his thick head.)


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